This invention relates to memory elements, and more particularly, to volatile memory elements that have elevated output voltages for integrated circuits such as programmable logic devices.
Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches) and are used to store data. Each memory element can store a single bit of data.
Volatile memory elements are often used to store configuration data in programmable logic devices. Programmable logic devices are a type of integrated circuit that can be customized in relatively small batches to implement a desired logic design. In a typical scenario, a programmable logic device manufacturer designs and manufactures uncustomized programmable logic device integrated circuits in advance. Later, a logic designer uses a logic design system to design a custom logic circuit. The logic design system uses information on the hardware capabilities of the manufacturer's programmable logic devices to help the designer implement the logic circuit using the resources available on a given programmable logic device.
The logic design system creates configuration data based on the logic designer's custom design. When the configuration data is loaded into the memory elements of one of the programmable logic devices, it programs the logic of that programmable logic device so that the programmable logic device implements the designer's logic circuit. The use of programmable logic devices can significantly reduce the amount of effort required to implement a desired integrated circuit design.
Conventional programmable logic device memory elements are powered at a positive power supply voltage. The positive power supply voltage that is used to power conventional programmable logic device memory elements is typically referred to as Vcc or Vcc-core and is the same power supply voltage used to power the core logic in the programmable logic device.
Integrated circuits such as programmable logic device integrated circuits that operate at low values of Vcc offer benefits over integrated circuits that operate at higher values of Vcc. For example, reductions in Vcc generally lead to reduced power consumption. Because of these benefits, the semiconductor industry is continually striving to produce processes and circuit designs that support reductions in Vcc. Previous generations of programmable logic devices operated at Vcc levels of 2.0 volts, 1.8 volts, and 1.5 volts. More recently, Vcc levels of 1.2 volts have been used in programmable logic devices. It is expected that future programmable logic devices will support Vcc levels of less than 1.2 volts (e.g., 1.1 volts or 1.0 volts).
The memory elements in a programmable logic device produce static output signals that reflect the configuration data that has been loaded into the memory elements. The static output signals drive the gates of n-channel and p-channel metal-oxide-semiconductor (MOS) transistors. Some of the transistors such as the n-channel transistors are used as pass transistors and are incorporated into multiplexers and other logic components. P-channel transistors are sometimes used as power-down transistors that prevent power from being applied to unused portions of an integrated circuit. Both n-channel and p-channel transistors operate poorly when they are driven at insufficient voltages. For example, if the gate of an n-channel pass transistor receives a voltage that is too low, the transistor will not turn on properly and will degrade logic signals passing through the transistor. If the gate of a p-channel power-down transistor is too low, the transistor will not turn off properly and will exhibit an undesirably large leakage current.
It would therefore be desirable to be able to provide programmable logic device integrated circuits that operate well at low core logic power supply levels.